1. Field of the Invention
This invention relates to magnetic particles for charging which electrostatically charge an image-bearing member used in recording processes that utilize electrophotography, applicable in copying machines, printers and facsimile machines. It also relates to a process for producing such magnetic particles, and a charging member, a process cartridge and an image-forming apparatus which have the magnetic particles.
2. Related Background Art
A number of methods are conventionally known as electrophotography. Copies or prints are commonly obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a visible image as a toner image, transferring the toner image to a transfer medium such as paper as occasion calls, and thereafter fixing the toner image to the transfer medium by heat and/or pressure. Toner particles that have not transferred to the transfer medium and have remained on the photosensitive member are removed from the photosensitive member through a cleaning step.
As means for charging the photosensitive member in such electrophotography, charging methods are available which utilizes corona discharging, called corotron or scorotron. Also, charging methods in which a charging member such as a roller, a fur brush or a blade is brought into contact with the photosensitive member surface to effect discharging in a narrow space in the vicinity of the contact area so as to cause ozone as less as possible have been brought forth and have been put into practical use.
In the charging method utilizing corona discharging, however, electrophotographic apparatus must be provided with a filter for capturing ozone, since ozone is generated in a large quantity especially when negative or positive corona is formed. This has involved problems that the apparatus have a large size and require a high running cost. Also, the method in which the charging member such as a blade or a roller is brought into contact with the photosensitive member to effect charging tends to cause problems that the toner melt-adheres onto the photosensitive member.
Accordingly, a method is also studied in which the charging member is used in the state it is only set close to the photosensitive member to avoid direct contact. Members for charging photosensitive members include the above roller, blade and brush, or members comprising a slender conductive plate-like material provided with a resistance layer. However, the charging carried out using any of such members has a problem that the distance between the charging member and the photosensitive member can be controlled with difficulty, and has been difficult to bring into practical use.
Accordingly, a technique is also studied in which what is called a magnetic brush is used as a charging member, which may apply a relatively small load to the photosensitive member during contact and holds magnetic particles with a magnet member. As charging methods making use of magnetic particles, two methods are proposed in combination of photosensitive members. One of them is a method in which a charge injection layer is provided as a surface layer of the photosensitive member and electric charges are directly injected through contact with this charge injection layer to charge the photosensitive member electrostatically. The other is a method in which an ordinary photosensitive member is used and discharging at microscopic gaps between magnetic particles and the surface of the photosensitive member is utilized.
With regard to magnetic particles used as a charging member, Japanese Patent Application Laid-open No. 59-133569 discloses that magnetic particles are coated with iron powder to make adjustment of resistance; and Japanese Patent Application Laid-open No. 6-295115, that resin layers containing conductive particles are formed on magnetic particle surfaces to make improvement for making environmental dependency of resistivity small and making magnetic particle surfaces less contaminable. Japanese Patent Application Laid-open No. 8-6355 further discloses a proposal that the running performance (durability) of a charging member is improved by mixing magnetic particles having smooth surfaces and magnetic particles having uneven surfaces, and Japanese Patent Application Laid-open No. 8-69149 discloses that magnetic particles having particle size distribution with a plurality of peaks are used to prevent the magnetic particles from surface contamination to achieve a long-term charging stability.
Techniques of applying a magnetic-brush charging assembly in the formation of full-color images are also disclosed. For example, Japanese Patent Application Laid-open No. 6-317969 discloses an apparatus having yellow, cyan, magenta and black developing assemblies for one photosensitive member and charging the photosensitive member through a magnetic brush. This, however, does not disclose any preferable magnetic particles at all in relation to the formation of color images.
As discussed above, in respect of magnetic particles serving as a charging member for charging photosensitive members, it is desired for them to be constituted more preferably. Also, in recent years, with progress of image input-output, processing and display techniques, hard copying/printing techniques which can be more simple and low-cost are desired, in the course of which away of solution is sought also on problems peculiar to instances where the magnetic particles are used and full-color images are formed.
As stated above, it is required to bring forth magnetic particles for charging that are more preferably constituted as a charging member. More specifically, it is sought to provide magnetic particles having stable charging performance and durable to long-term service when a charging assembly making use of magnetic particles for charging is continuously used, and also to provide a charging member, a process cartridge and an image-forming apparatus which employ such magnetic particles. In the formation of full-color images, too, it is sought to provide magnetic particles that can contribute to the formation of sharp images over a long period of time, and also to provide a charging member, a process cartridge and an image-forming apparatus which employ such magnetic particles.
The present invention was made from the above point of view. Accordingly, an object of the present invention is to provide magnetic particles for charging which have a superior service durability, and a charging member, a process cartridge and an image-forming apparatus which employ such magnetic particles.
Another object of the present invention is to provide a process cartridge and an image-forming apparatus which are less causative of abrasion of photosensitive members.
Still another object of the present invention is to provide magnetic particles for charging which can contribute to the formation of sharp images over a long period of time in the formation of full-color images, too, and a charging member, a process cartridge and an image-forming apparatus which employ such magnetic particles.
A further object of the present invention is to provide a process for producing magnetic particles for charging which have a superior service durability, are less causative of abrasion of photosensitive members and can contribute to the formation of full-color images stable over a long period of time.
To achieve the above objects, the present invention provides magnetic particles for charging which are to be rubbed against an image-bearing member on which an electrostatic latent image is to be formed, to charge the image-bearing member electrostatically; the magnetic particles for charging comprising;
magnetic particles;
first surface coat layers containing a first surface coating agent and covering the surfaces of the magnetic particles; and
second surface coat layers containing a second surface coating agent and covering the surfaces of the magnetic particles covered with the first surface coat layers;
the magnetic particles for charging containing magnetic particles for charging which have a maximum chord length of 5 xcexcm or larger, and the magnetic particles for charging which have a maximum chord length of 5 xcexcm or larger having a minor-axis length/major-axis length standard deviation of 0.08 or more.
The present invention also provides a process for producing magnetic particles for charging which are to be rubbed against an image-bearing member on which an electrostatic latent image is to be formed, to charge the image-bearing member electrostatically; the process comprising the steps of;
preparing magnetic particles by pulverization;
coating the pulverized magnetic particles with a first surface coating agent to form first surface coat layers on their surfaces; and
coating the magnetic particles on which the first surface coat layers have been formed, with a second surface coating agent to form second surface coat layers on the former""s surfaces.
The present invention also provides a charging member which is to be rubbed against an image-bearing member on which an electrostatic latent image is to be formed, to charge the image-bearing member electrostatically; the charging member comprising a magnet member having a conductor to which a voltage is to be applied, and magnetic particles for charging which are held on the magnet member by the action of magnetism;
wherein the magnetic particles for charging comprise;
magnetic particles;
first surface coat layers containing a first surface coating agent and covering the surfaces of the magnetic particles; and
second surface coat layers containing a second surface coating agent and covering the surfaces of the magnetic particles covered with the first surface coat layers;
the magnetic particles for charging containing magnetic particles for charging which have a maximum chord length of 5 xcexcm or larger, and the magnetic particles for charging which have a maximum chord length of 5 xcexcm or larger having a minor-axis length/major-axis length standard deviation of 0.08 or more.
The present invention still also provides a process cartridge which is detachably mountable on an image-forming apparatus in which an electrostatic latent image formed on an image-bearing member is rendered visible by the use of a toner and a toner image formed by rendering the electrostatic latent image visible is transferred to a transfer medium to form an image; the process cartridge comprising;
a charging means for charging the image-bearing member electrostatically by bringing a charging member into contact with the image-bearing member; the charging member comprising a magnet member having a conductor to which a voltage is to be applied, and magnetic particles for charging which are held on the magnet member by the action of magnetism; and
at least one means selected from the group consisting of i) the image-bearing member on which the electrostatic latent image is to be formed, ii) a developing means for rendering the electrostatic latent image formed visible by the use of a toner, and iii) a cleaning means for removing the toner, remaining on the image-bearing member after the toner image formed by rendering the electrostatic latent image visible has been transferred to the transfer medium;
wherein the magnetic particles for charging comprise;
magnetic particles;
first surface coat layers containing a first surface coating agent and covering the surfaces of the magnetic particles; and
second surface coat layers containing a second surface coating agent and covering the surfaces of the magnetic particles covered with the first surface coat layers;
the magnetic particles for charging containing magnetic particles for charging which have a maximum chord length of 5 xcexcm or larger, and the magnetic particles for charging which have a maximum chord length of 5 xcexcm or larger having a minor-axis length/major-axis length standard deviation of 0.08 or more.
The present invention further provides an image-forming apparatus comprising;
an image-bearing member on which an electrostatic latent image is to be formed;
a charging means for charging the image-bearing member electrostatically by bringing a charging member into contact with the image-bearing member; the charging member comprising a magnet member having a conductor to which a voltage is to be applied, and magnetic particles for, charging which are held on the magnet member by the action of magnetism;
an exposure means for exposing to light the surface of the image-bearing member charged by the charging means, to form an electrostatic latent image;
a developing means for rendering the electrostatic latent image formed visible by the use of a toner to form a toner image; and
a transfer means for transferring the toner image formed, to a transfer medium;
wherein the magnetic particles for charging comprise;
magnetic particles;
first surface coat layers containing a first surface coating agent and covering the surfaces of the magnetic particles; and
second surface coat layers containing a second surface coating agent and covering the surfaces of the magnetic particles covered with the first surface coat layers;
the magnetic particles for charging containing magnetic particles for charging which have a maximum chord length of 5 xcexcm or larger, and the magnetic particles for charging which have a maximum chord length of 5 xcexcm or larger having a minor-axis length/major-axis length standard deviation of 0.08 or more.